The (supposed) space drive at issue bounces microwaves around a specially shaped
chamber, and in the process is said (somehow) to produce a net thrust in one direction. The nature of the impossibility? That the drive -- if it works as advertised -- violates conservation of momentum.

Physics has gone a long time with every bit of evidence showing momentum is conserved. Always.

Space drive: old school

Consider a traditional, chemical rocket. It relies upon Newton's third law of motion: that every action produces an equal but opposite reaction. The action: exhaust gasses streaming out behind the rocket. The reaction: the rocket itself moves forward.

Newer, ion thrusters forgo chemistry but exploit the same action/reaction mechanism. That is, they use electromagnetic fields to propel Xenon ions. In both cases, matter ("reaction mass") is expelled from the spacecraft. Momentum is conserved.
If the "emdrive" or the related variant, the "Cannae drive," does work, it would mean spacecraft could operate solely with solar energy, not expelling any reaction mass. If a satellite or space probe need not carry and expend fuel, it could be much smaller and/or operate much longer.

But NASA reported that the "space drive" worked. NASA! Surely they took precautions, checked and rechecked everything. Well, no -- because (notwithstanding the headlines) NASA didn't perform the study. A few people at NASA did the study.

How does the "impossible" drive work? We read:

"Test results indicate that the RF resonant cavity thruster
design, which is unique as an electric propulsion device, is
producing a force that is not attributable to any classical
electromagnetic phenomenon and therefore is potentially
demonstrating an interaction with the quantum vacuum virtual
plasma."

Only what is the"quantum vacuum virtual
plasma?" Not a standard term in physics.
Might these inventers have discovered some new, conservation-of-momentum-defying physics? That would certainly be interesting. But (in Carl Sagan's words, not mine), extraordinary claims require extraordinary evidence.

The thrust claimed for the new drive was tiny: about ten micro-newtons. (In customary units, that's 0.000036 ounces of force.) Such a small value makes problematical that the test of this supposed space drive wasn't done in a vacuum. Perhaps the tiny effect will be traceable to an errant air current.

Even if that tiny thrust can be replicated in a vacuum chamber, attributing the result to new physics should be the last resort. Consider, for example, that physical objects of nonuniform composition or shape radiate heat asymmetrically, producing a tiny thrust --

Billions of miles (and still going)

As appears to be the resolution of the longstanding Pioneer Anomaly. As exciting as a long-range-only, theretofore unobserved fine-tuning to our understanding of gravity would have been, nothing of the sort was needed to explain Pioneer 10 and 11 probes following what were slightly unexpected courses as they take leave of the Solar System.

Heat, aka the random jitter of molecules, results in "thermal radiation": aka, warm objects emit photons. (That's how night vision goggles work, by exploiting the infrared emissions of objects not observable in the wavelengths to which our eyes are sensitive.) Photons, though massless, carry momentum in proportion to their frequency. When those emissions are asymmetric, there's a tiny net force vector ...

NASA did not conduct the study; a
research team within NASA did. So the findings are not quite as
authoritative as they appear to be, notes io9. This differentiation has
posed a problem in the past, most recently in March, when news
circulated that NASA did a study predicting when society would end. NASA funded the climate model used in the study but was not involved in the research.

What to read?

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Featured Post: A Milestone

On October 16, 2007, Fleet of Worlds was first published. That is: ten years ago to the day. Larry and Ed at 2015 Nebula weekend This...

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About Me

I'm a physicist and computer scientist (and an MBA, of less relevance to most of these posts). After thirty years in industry, as everything from individual technical contributor to senior vice president, I now write full-time. Mostly I write science fiction and techno-thrillers, now and again throwing in a straight science or technology article.